1. Field of the Invention
The invention relates to a compensation device and a power transmission system using such a compensation device. In particular, the invention relates to such a compensation device which comprises a transformerless reactive series compensator.
2. Description of the Related Art
Compensation devices are typically used in power transmission systems in order to increase power transmission capacity and to make the power more stable. Normally, such a compensation device is connected in series with power transmission lines including a circuit breaker and includes at least one compensator unit. Each compensator unit includes a transformerless reactive series compensator which comprises a DC capacitor and a single-phase inverter constituted by self arc-suppressing semiconductors.
More specifically, in the compensation unit the respective inverter is connected to a respective DC capacitor. In such a compensation device, before a normal operation can start, the DC capacitor must be pre-charged to a specified voltage which is conventionally performed by an additional pre-charging circuit or by a line current which flows through a transmission line.
The invention in particular relates to the problem of how the compensation devices can efficiently pre-charge the at least one DC capacitor with a pre-charging circuit of a simple hardware configuration.
Several different examples of compensation devices have been used in power transmission systems in recent years to provide more stable and higher efficiency power transmission systems. Such power transmission systems are called flexible AC transmission systems (so-called FACTS). In the compensation devices, semiconductor devices have been conventionally applied. Such semiconductor devices can comprise self arc-suppressing semiconductors like gate-turn-off thyristors (so-called GTOs) and gate-commutated turn-off thyristors (so-called GCTs). In addition, insulated gate bipolar transistors (so-called IGBTs) may be employed. Such components have been applied to power converters, e.g., the inverters of the compensation devices, and it is invisaged that their application will be more widespread in the future in order to realize more stable power transmission systems.
There is one compensation device which comprises a transformerless reactive series compensator (so-called TLRSC) to be used in the FACTS systems.
FIG. 9 shows a power transmission system, comprising two AC power systems 1a, 1b coupled to each other through power transmission lines 2a, 2b, a compensation device 3, namely a TL-RSC 3, and a circuit breaker 4 provided between the AC power system 1b and the power transmission line 2b. The TLRSC 3 is provided between the power transmission lines 2a and 2b not only to increase the power transmission capacity but also to make it more stable. In addition, the TL-RSC 3 is directly connected in series with the transmission lines 2a, 2b at two output terminals 3a, 3b without any transformer.
FIG. 10 shows a detailed configuration of the TL-RSC 3 as known from the following two prior art references, "Transformerless Reactive Series Compensators with Voltage Source Inverters", Proceedings of the Power Conversion Conference (PCC)--Nagaoka 1997, pp. 197-202 and EP 98 106 780.4.
In FIG. 10, self arc-suppressing semiconductors are designated by 5a to Ed, free-wheeling diodes connected in anti-parallel with each of the self arc-suppressing semiconductor 5a to Ed are designated by 6a to 6d, a single-phase inverter which consists of the self arc-suppressing semiconductors 5a to Ed and the free-wheeling diodes 6a to Ed, is designated by 7, a DC capacitor connected to the single-phase inverter 7 is designated by 8, filter reactors are designated by 9a and 9b, a filter capacitor is designated by 10, a filter circuit which consists of the filter reactors 9a, 9b and the filter capacitor 10 is designated by 11 and an AC switch is designated by 12. Furthermore, a compensator unit of the compensator device TL-RSC 3 is designated by 13 and two output terminals of the compensator unit 13 are designated by 14a and 14b.
In FIG. 10, the self arc-suppressing semiconductor 5a is shown as being separate from the free-wheeling diode 6a. However, in recent years, reverse-conducting self arc-suppressing semiconductors which integrate both functions of the self arc-suppressing semiconductor 5a and the free-wheeling diodes 6a in the same package have been developed. When the reverse-conducting self arc-suppressing semiconductors are applied to the single-phase inverter 7, the free-wheeling diodes 6a to Ed are not necessary in FIG. 10.
Whilst in FIG. 10, the compensator device TL-RSC 3 consists of one compensator unit 13, FIG. 11 shows a compensator device consisting of several compensator units denoted 13a, 13b, 13c. Such a configuration can have a much more powerful compensatable capacity. Here, the compensator device TL-RSC 3 consists of several cascaded compensator units 13a to 13c connected in series at their output terminals 14a and 14b of each compensator unit 13a, 1ab, 13c. Of course, each of the compensator units 13a to 13c has the same configuration as shown in FIG. 10. Conventionally, typically up to 10 such compensator units may be connected in series.
As shown in FIG. 10, the single-phase inverter 7 is connected in series and indirectly with the power transmission lines 2a, 2b without any transformer. That is, the inverter 7 is coupled to the power transmission lines 2a, 2b via the filter circuit 11 inserted in series to the transmission lines 2a, 2b. The filter circuit 11 essentially suppresses harmonic distortions which result from the single-phase inverter 7 being operated by a pulse width modulation (so-called PWM) control (as is well known, the PMW control is applied for switching the inverter 7 on/off over predetermined time-intervals). When the influence from the single-phase inverter 7 on the transmission lines 2a, 2b (i.e. regarding the impedance matching and/or the spikes caused on the transmission lines by the PWM control) is very small, especially in the case of high PWM frequencies, the filter circuit 11 may be eliminated. Thus, a separated configuration of the filter reactors 9a, 9b is not essential and only one of them may be sufficient.
The AC switch 12 is defined as a switch which opens and closes for connecting or disconnecting the terminals 14a, 14b, at predetermined turn-on/turn-off timings and which can also pass an AC current when closed (i.e. in an on-state). EP 98 106 780.4 discloses the control of such an AC switch 12 for start-up and shut-down operations of a compensator device TL-RSC 3.
Although the use of a compensator device with a TL-RSC 3 has been generally proposed for new FACTS systems, no actual operable circuit has been presented up to now due to some intrinsic problems. During its normal PWM operation the TL-RSC 3 controls the line current flowing through the transmission lines 2a, 2b and the single-phase inverter 7 generates an output voltage by using a voltage of the DC capacitor 8.
However, before the start of a normal PWM operation the voltage of the DC capacitor 8 is zero and therefore a pre-charging operation of the DC capacitor 8 is necessary. Thus, the realization of a workable TL-RSC 3 depends heavily on the provision of effective pre-charging techniques, in particular with respect to the cost, size and weight of the individual components used for the pre-charging, as will be explained hereinafter.
Firstly, a half-bridge circuit of an inverter for three phases including a pre-charging circuit for the DC capacitor 8 is shown in FIG. 12. As is seen in FIG. 12, the circuit configuration is a parallel one, i.e. the pre-charging of the DC capacitor 8 is performed by diverting a part of the current flowing through each phase (3 phase line) of the power transmission lines 2 to the respective half-bridge circuit and thus to the DC capacitor. The pre-charging circuit makes it possible to charge the DC capacitor 8 with current from the transmission line 2.
A pre-charging circuit as in FIG. 12 is illustrated in "Mitsubishi Denki Gihou, Vol. 63, No. 10, 1989, pp. 41". Here, the inverter has been applied to an active filter in order to decrease the harmonic distortion in the transmission line 2. As is Seen in FIG. 12, as the pre-charging circuit a switch 15a is connected in series to a respective pre-charging resistor 16 and another switch 15b is connected in parallel across the switch 15a and the pre-charging resistor 16.
During the pre-charging operation, i.e. before the normal PWM operation of the active filter can begin, the switch 15a is on and the switch 15b is off. Thus, it is possible to get power (basically current) through the pre-charging resistor 16 from the transmission line 2 and to pre-charge tee DC capacitor 8. The pre-charging resistor 16 works as a current limiting resistor.
After the pre-charging operation period, the pre-charging resistor 16 is shorted by the switch 15b turning on and, at the same time, the switch 15a should be opened.
When such a parallel-type pre-charging circuit as used in FIG. 12 for an active filter is applied to a compensator device TL-RSC 3, which especially has such a multiple configuration as illustrated in FIG. 11, many pre-charging resistors 16 (3 for each compensator unit) and many auxiliary switches 15a, 15b (2 for each compensator unit) are necessary. In addition, during the normal operation of the TL-RSC 3, the line current flows through the switch 15b. When applying the parallel-type pre-charging circuit of FIG. 12 to an active filter, then a conducting current which flows through the switch 15b is a current to compensate the distorted current in the power transmission systems (not shown in FIG. 12). Therefore, the conducting current of the switch 15b is in, the case of the Active Filter comparatively small.
However, when applying the pre-charging circuit of the parallel-type to a compensator device TL-RSC 3 including a transformerless reactive series compensator, on the other hand, the on-state power loss caused by the switch 15b is extremely high due to the high currents and voltage occurring in a power transmission system.
Secondly, in a compensator unit 13, a filter circuit 11 is employed which in particular comprises a filter capacitor 10. The capacitance of the filter capacitor 10 is designed to decrease harmonic distortion caused from the single-phase inverter 7 driven by the PWM control. On the other hand, the filter capacitor 10 is connected in series to the transmission lines 2a, 2b during the above-mentioned pre-charging operation (during the pre-charging the AC switch 12 is open) and the capacitance of the filter capacitor 10 is not matched in any way to the series connection with the transmission line 2a, 2b. Therefore, the series-type compensator device 3 in FIG. 10 has another disadvantage, namely that the capacitance of the filter capacitor 10 has no impedance-matching to the transmission lines. Therefore, if such a parallel-type pre-charging circuit as in FIG. 12 is applied to the series-type compensator device TL-RSC 3 as in FIG. 10, the filter capacitor 10 may be overcharged during the pre-charging because of the unmatched capacitance of the filter capacitor 10 of the filter circuit 11.
As mentioned above, there are some aspects to consider when the conventional pre-charging technique is applied to the TL-RSC 3. Namely, a lot of pre-charging resistors 16 are necessary for each compensator unit 13. Additionally, many switches 15a, 15b to connect or to disconnect the respective pre-charging resistors 16 are also necessary. In addition, comparatively large capacity cooling systems are necessary for the switches 15b to pass the line current due to the high currents and voltages occurring in a power transmission system. Furthermore, in order to solve the overvoltage problem of tie filter capacitor 10, the capacitance of the filter capacitor 10 must be increased or the filter capacitor 10 must be Equipped with a voltage limiting device. These prerequisites require large-size and expensive components. Therefore, they make the TL-RSC 3 itself larger and more expensive, i.e. the parallel-type pre-charging circuit using the line current for the pre-charging of the DC capacitor cannot be applied to the series-type compensator device.